/Engine/Source/Runtime/Engine/Public/Streaming/LevelStreamingDelegates.h
Notifications on when work actually beings on making a level visible or invisible, so profiling tools can measure the difference between time spent waiting to be added to the world and time spend actively working.
static [TMulticastDelegate](API\Runtime\Core\Delegates\TMulticastDelegate)< void([UWorld](API\Runtime\Engine\Engine\UWorld) *, const [ULevelStreaming](API\Runtime\Engine\Engine\ULevelStreaming) *, [ULevel](API\Runtime\Engine\Engine\ULevel) *LoadedLevel)> OnLevelBeginMakingVisible;